1. Field of the Invention
The present invention relates to a cooling air supply system used for rotors operating at a high temperature and requiring cooling air.
2. Description of the Related Art
FIG. 5 shows a typical configuration of a cooling air passage used in a gas turbine rotor.
In the machines having rotors operating at a high temperature, such as gas turbines, the rotors are cooled by supplying cooling air thereto. For example, rotor blades of gas turbines which contact a high temperature combustion gas are cooled by supplying cooling air to the cooling air passages formed within the blade in order to increase durability of the blades. Usually, cooling air is supplied to the rotor blade through axial cooling air passages formed in the rotating shaft of the turbine.
In FIG. 5, reference numeral 10 designates a gas turbine rotor as a whole. Rotor 10 includes a rotor shaft 11 and a plurality of turbine disks 13 coupled to the shaft 11 (in FIG. 5, only one turbine disk is shown). Rotor blades 15 for receiving energy of high temperature combustion gas in order to rotate the rotor 10 are embedded on the outer periphery of the disks 13.
As shown in FIG. 5, a cylindrical sleeve 31 is disposed around the outer periphery of the rotor shaft 11. The cylindrical sleeve 31 is attached to the side face of the rotor disk 13 and rotates together with the rotor shaft 11. The cylindrical sleeve 31 has an inner diameter larger than the outer diameter of the shaft 11 and the clearance between the inner surface of the sleeve 31 and the outer surface of the shaft 11 forms an annular cooling air passage 30. An end of the cooling air passage 30 opposite to the turbine disk 13 opens to a cooling air supply chamber 20 and acts as an annular air inlet opening 33.
The cooling air supply chamber 20 is formed in a casing 50 accommodating the rotor 10 as an annular chamber surrounding the open end (i.e., the annular air inlet opening 33) of the sleeve 31. Labyrinth seals 51 and 53 of a known type are disposed on both axial ends of the cooling air supply chamber 20 to prevent cooling air in the chamber 20 from escaping through the clearances between a stationary member (i.e., the casing 50) and a rotating member (i.e., the rotor 11 and the cylindrical sleeve 31).
Cooling air is supplied from a pressurized air source (typically, from the discharge of the air compressor of the gas turbine) to the cooling air supply chamber 20 through a cooling air piping 23. From the cooling air supply chamber 20, cooling air flows into the annular air inlet opening 33 formed around the outer periphery of the shaft 11 and flows through the cooling air passage 30 in the axial direction to the turbine disk 13. At the turbine disk 13, a part of the cooling air is supplied to the root portions of each of the blades 15 through radial air passages 13a disposed in the turbine disks 13. Cooling air supplied to the roots of the blade 15, then, flows through a blade cooling air passage formed within the blades (not shown) to cool the material of the blade 15 and discharged from outlet holes disposed on the leading edges or trailing edges of the blades 15. The remaining portion of cooling air (i.e., cooling air not flowing into the radial cooling air passage 13a) is supplied to other turbine disks (not shown) through an axial passage 13b perforating through the turbine disk 13 in order to cool the rotor blades of other rotors.
When cooling air flows through the cooling air passage 30, a velocity component tangential to the outer periphery of the shaft is given to the flow of cooling air through the contact with the outer surface of the rotor shaft 11 which rotates at a high speed and cooling air in the passage 30 flows in a spiral flow path around the shaft 11 towards the turbine disk 13. This causes a power loss at the rotor shaft 11 by an amount equal to the kinetic energy, corresponding to the tangential velocity component, given to cooling air at the air inlet 33 of the cooling air passage 30.
In order to reduce the power loss at the rotor shaft due to the tangential velocity imparted to cooling air, tangential cooling air nozzles are used in some cases. In the cooling air system equipped with tangential cooling air nozzles, air nozzles injecting cooling air to a direction tangential to the outer peripheral of the rotor shaft are disposed in the casing 50 near the inlet 33 of the cooling air passage 30. Since cooling air is ejected from the tangential air nozzles in the direction tangential to the outer periphery of the shaft 11, the tangential velocity component is imparted to cooling air before it flows into the cooling air passage 30. Therefore, the power loss at the rotor shaft due to the tangential velocity component is largely reduced.
The tangential air nozzles consist of a number of nozzle members having aerofoil cross sections and are disposed radially around the rotor shaft 11 at the air inlet 33 of the cooling air passage 30 and air passages are formed by the clearance between the nozzle members. Usually, the tangential nozzles are formed as expansion nozzles, i.e., the air passages of the nozzles are designed in such a manner that cooling air passing through the nozzles expands in the air passages in the nozzles and is ejected in the tangential direction at a predetermined speed. The pressure difference across the nozzles, i.e., the pressure difference between the pressure of the air in the cooling air supply chamber 20 and the pressure of air at the air inlet 33 of the cooling air passage 30 is determined in such a manner that the magnitude of the velocity of cooling air leaving the nozzles is substantially the same as the peripheral speed of the rotating shaft 11 during the operation of the gas turbine. Since cooling air is ejected from the nozzle in the tangential direction at the speed the same as the peripheral speed of the shaft 11, a difference in the magnitude of the tangential velocity components does not occur when cooling air and the rotor shaft contact each other and power loss at the rotor does not occur.
However, problems occur when nozzle members having aerofoil cross sections are used for tangential air nozzles. When the aerofoil type nozzle members are used, the tangential nozzles are formed by assembling separately manufactured nozzle members by disposing the respective nozzle members around the air inlet of the cooling air passage and securing them to the stationary members in the cooling air supply chamber by welding or brazing. Alternatively, all the tangential nozzles may be made of a one-piece annular member including the aerofoil nozzle members arranged radially therein and may be formed by casting or by machining an annular shaped material by, for example, an electric discharge method.
However, since the shape of the aerofoil nozzle members and the arrangement thereof is complicated, manufacturing and assembly of the tangential nozzles requires many man-hours and, therefore, is costly. Further, when welding or brazing are used for assembling the nozzles, distortion of the nozzle members due o the high temperature used during the assembly may occur. This causes inaccuracy of the direction of cooling air ejected from the nozzles.
Further, if the aerofoil type nozzle members are used, it is necessary to arrange the respective nozzle members in a direction tangential to the outer periphery of the shaft, i.e., the respective nozzle members must be arranged around the outer periphery of the shaft in such a manner that each of the nozzle members is arranged on a plane perpendicular to the axis of the shaft and inclines at a predetermined angle with respect to a radius of the shaft. When the outlets of the nozzles are located in proximity of the outer periphery of the shaft the inclination of the nozzle members becomes larger and, therefore, the manufacturing and assembling of the tangential nozzles becomes more difficult.
In the tangential air nozzles explained above, cooling air is ejected from the nozzles only in the tangential direction. In other words, cooling air ejected from the nozzles does not have an axial velocity component (i.e., a velocity component in the direction parallel to the axis of the shaft). However, cooling air ejected from the nozzles flows in the cooling air passage in the axial direction. Therefore, cooling air ejected from the nozzles must change its flow direction toward the axial direction when it flows into the cooling air passage. This change in the flow direction causes a pressure drop in the flow of cooling air. Therefore, when the tangential air nozzles are used, the pressure of the air in the cooling air supply chamber must be increased in order to obtain a required amount of cooling air flow. As explained before, since cooling air is supplied from the gas turbine air compressor, if the cooling air supply pressure is increased, the power loss in the gas turbine as a whole increases due to an increase in the power consumption of the air compressor.
Theoretically, it is possible to reduce the power loss caused by the change in the cooling air flow direction by imparting an axial velocity component, in addition to the tangential velocity component, to cooling air ejected from the tangential nozzles. However, in order to impart the axial velocity component to cooling air, the nozzles must be inclined to axial direction in addition to the tangential direction. When the aerofoil type nozzle members are used for the air nozzles, it is extremely difficult to incline the nozzles to the tangential direction and to the axial direction simultaneously.
In view of the problems as set forth above, the objects of the present invention is to provide a cooling air supply system for a rotor in which the air nozzles can be manufactured at low cost and with a high accuracy while avoiding the pressure loss in the cooling air due to a change in the flow direction.
One or more of the objects as set forth above are achieved by a cooling air supply system for a rotor, according to the present invention, comprising a cooling air passage disposed in a rotor shaft and extending in a direction along the axis of the shaft, the cooling air passage being provided with an air inlet opening disposed around the outer periphery of the shaft, an annular cooling air supply chamber surrounding the shaft at the portion the air inlet opening is located, the cooling air supply chamber being connected to a pressurized air source, a plurality of cooling air nozzles for injecting cooling air in the cooling air supply chamber into the air inlet opening of the cooling air passage, wherein the cooling air nozzles are formed as straight passages having circular cross sections and having air outles and inlets thereof opening near the air inlet of the cooling air passage and to the cooling air supply chamber, respectively, the straight passages are formed as through holes perforating a stationary annular member surrounding the periphery of the rotor shaft.
According to the present invention, the cooling air nozzles are formed as a plurality of straight air passages having circular cross sections perforated through the annular stationary member. Therefore, the cooling air nozzles can be easily manufactured at low cost, for example, by drilling through holes in the stationary member. Further, since the cooling air nozzles can be formed by drilling, high accuracy of the dimensions and directions of the cooling air nozzles can be achieved without increasing the manufacturing cost. Thus, according to the present invention, the cooling air nozzles can be manufactured accurately and at low cost.
Further, since the cooling air nozzles are formed as straight air passages perforating through the stationary member, the freedom for the arrangement of the nozzles remarkably increases compared with the case where the aerofoil type nozzle members are used. Therefore, the straight air passages can be easily arranged in such a manner that the straight air passages extend in a direction tangential to the outer periphery of the rotor shaft and, at the same time, incline at an angle from a plane perpendicular to the axis of the rotor shaft. Therefore, the cooling air nozzles can be easily arranged so that cooling air leaving the nozzle has an axial velocity component as well as a tangential velocity component in order to avoid a pressure loss in the cooling air due to the change in the flow direction of the cooling air.